1. Technical Field
This application relates to managing read caching.
2. Description of Related Art
Computer systems may include different resources used by one or more host processors. Resources and host processors in a computer system may be interconnected by one or more communication connections. These resources may include, for example, data storage devices such as those included in the data storage systems manufactured by EMC Corporation. These data storage systems may be coupled to one or more servers or host processors and provide storage services to each host processor. Multiple data storage systems from one or more different vendors may be connected and may provide common data storage for one or more host processors in a computer system.
A host processor may perform a variety of data processing tasks and operations using the data storage system. For example, a host processor may perform basic system I/O operations in connection with data requests, such as data read and write operations.
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data in the device. In order to facilitate sharing of the data on the device, additional software on the data storage systems may also be used.
The data storage system may utilize a variety of different techniques such as caching. With caching, the data storage system may cache data from the physical disk drives within its storage cache. As known in the art, the cache used in the data storage system may be implemented using a fast, volatile memory, such as RAM (random access memory), particularly dynamic RAM (DRAM).
It should be noted that a data storage system may include multiple storage processors (also referred to as storage controllers) storing data to a same set of storage devices. Each of the storage processors may have its own cache so that cached data may be mirrored in the caches of the storage processors. Multiple storage processors may be desirable for use in providing fault tolerance, higher throughput, and the like.
In a particular example, as is known in the art, large host computer systems require large capacity data storage systems. These large computer systems generally include data processors which perform many operations on data introduced to the computer system through peripherals including the data storage system. The results of these operations are output to peripherals, including the storage system.
In accordance with the example, one type of data storage system is a magnetic disk storage system. Here a bank of disk drives and the computer system are coupled together through an interface. The interface includes “front end” directors (or controllers) and “back end” disk directors (or controllers, also known as rear end directors or disk directors). The interface operates the directors in such a way that they are transparent to the computer. That is, data is stored in, and retrieved from, the bank of disk drives in such a way that the computer system merely thinks it is operating with one large memory. One such system is described in U.S. Pat. No. 5,206,939, entitled “System and Method for Disk Mapping and Data Retrieval”, inventors Moshe Yanai, Natan Vishlitzky, Bruno Alterescu and Daniel Castel, issued Apr. 27, 1993, and assigned to the same assignee as the present invention.
As described in such U.S. patent, the interface may also include, in addition to the front-end directors and disk directors, an addressable global cache memory. The global cache memory is a semiconductor memory connected to all of the front end directors and back end directors and is provided to rapidly store data from the computer system before storage in the disk drives, and, on the other hand, store data from the disk drives prior to being sent to the computer. The cache memory being a semiconductor memory, as distinguished from a magnetic memory as in the case of the disk drives, is much faster than the disk drives in reading and writing data.
A read cache may be used for staging/prefetching data for fulfilling data read requests received from a host. For example, a data storage system having a read cache function allows a cache memory to receive data from a disk address designated by a read request. When another read request is issued to read the data from the disk address, the data is read not from the disk but instead from the cache memory for transfer to an interface. Because it is not necessary to wait for the disk to rotate every time to reach appropriate positions or otherwise wait for disk access, high-speed data transmission is effected.
In an example, a data storage system has a pair of storage processors connected to an array of disk drives. For example, such a system is disclosed in U.S. Pat. No. 5,922,077, which is hereby incorporated by reference herein, and which describes a dual data storage controller system in which the controllers are connected to one another by a peer-to-peer communication link. Each data storage controller is connected to a fibre channel loop in connection with each of the disk drives in the disk array. Fail-over switches provide each data storage controller with a means for connecting to either one of the fibre channel loops.
Each storage processor has its own cache memory and the two storage processors may be configured to communicate with each other through a Cache Mirroring Interface (CMI) bus in the peer-to-peer communication link in order to maintain cache coherency. In particular, the CMI bus enables a copy of data to be available on both storage processing units before a disk write operation is complete. In this system, a first storage processing unit has a first CMI interface circuit, a second storage processing unit has a second CMI interface circuit, and the first and second CMI interface circuits connect to each other through the CMI bus.
The nature of non-volatile, vibration-free, small size and low power consumption has made flash memory an excellent component to be utilized in various flash storage devices. Flash storage devices are widely used as memory storage for computer and consumer system products such as notebook, desktop computer, set top box, digital camera, mobile phone, PDA and GPS etc. The increasing demand for more storage in these products has driven the need to expand the capacity of the flash storage devices.